Recently, optical communication using light as a medium for information transmission to transmit large-size information and high-speed information communication has been popularized. Recently, it is possible to easily convert an electrical signal of 5 Gbps into laser light, using a semiconductor laser diode chip having width and length of about 0.3 mm, and to easily convert an optical signal transmitted through an optical fiber into an electrical signal, using a semiconductor light receiving element. Light is an energy wave having very special characteristics, and in order for several lights that simultaneously exist in any one region to interact with each other, lights to be interacted with each other need to have the same wavelength, or phases of lights need to be matched to each other, and propagation directions thereof need to coincide with each other. As such, since light has very deteriorated coherence therebetween, WDM (wavelength division multiplexing) type optical communication simultaneously transmitting light having various wavelengths through one optical fiber has been preferred. The WDM type optical communication is one of very economical communication methods in that it may allow the optical fiber, which is a transmission medium of a signal, to be shared, thereby decreasing costs caused by an optical fiber routing.
For the WDM type optical communication, there is proposed a package type where a high-speed laser diode chip is embedded in a TO can-type small package. FIG. 1 illustrates an example of a stem structure of a TO can-type package for ultrahigh-speed communication according to the related art. As illustrated in FIG. 1, the To can-type package according to the related art is manufactured by a package method in which a laser diode chip, and the like are attached to a stem manufactured by punching a through hole in a metal substrate and inserting a metal electrode pin sealed with glass into the through hole, and the electrode pin aria the laser diode chip are electrically connected to each other by a gold (Au) wire.
According to the related art, a method for directly connecting the laser diode chip and the electrode pin by the Au wire is mainly used. However, as the TO can-type package recently became high speed, a problem of transmission signal distortion by the Au wire occurs, so a method for minimizing distortion of a ultrahigh-speed radio frequency (RF) signal by a length of the Au wire by attaching an impedance-matched RF substrate between the laser diode chip and the electrode pin, attaching the laser diode chip onto the RF substrate, and then electrically connecting the RF substrate and the laser diode chip to each other by the Au wire, is used.
Meanwhile, FIG. 1 illustrates a stem structure applied to a ultrahigh-speed signal in 10 Gbps class which is conventionally commercially available. In FIG. 1, the RF substrate is manufactured by attaching an AlN substrate formed of a AlN material and manufactured in a RF stripe pattern to a header of the stem, wherein after the laser diode chip is attached to one side of the AlN substrate, the laser diode chip and a signal line on the RF substrate are electrically connected to each other by the Au wire.
In the structure illustrated in FIG. 1, an attachment position of the laser diode chip is regardless of a thickness of the AlN substrate, and the AlN substrate is manufactured by attaching an AlN RF substrate having a predefined thickness to a position of the header. In this structure, a thickness of the RF substrate may be arbitrarily defined.
FIG. 2 illustrates an example of a side of a TO can-type laser diode package in which a thermoelectric element commonly used currently is embedded.
FIG. 2 illustrates an example in which a thermoelectric element 700 is disposed on a bottom of a stem 800, and a laser diode chip 100, a lens 500, and a 45° reflection mirror 600 are disposed on an upper plate of the thermoelectric element 700. A height to which laser light of the laser diode chip 100 is emitted should coincide with a central optical axis height of the lens 500, and typically, a size of the lens 500 is about 1 mm. Therefore, the central optical axis height of the lens 500 is about 0.5 mm. If the size of the lens 500 is further decreased, an aperture of the lens 500 is decreased. As a result, since it is difficult to collect enough laser light, it is difficult to decrease the size of the lens 500 to 0.8 mm or less even in the case in which the size of the lens 500 becomes smaller. Therefore, a height of a central axis of the lens 500 is 0.4 mm or more. Since the laser diode chip 100 typically has a thickness of about 0.1 mm, a height of a sub-mount 200 on which the laser diode chip 100 is put is typically about 0.4 mm in order to coincide a height of a laser emission point of the laser diode chip 100 with the central axis of the lens 500. However, it is difficult to manufacture a ultrahigh-speed signal transmission line capable of performing ultrahigh-speed communication using the sub-mount 200 having the thickness of 0.4 mm. Meanwhile, reference numeral 810 of FIG. 2 denotes a through hole into which the metal electrode pin is to be inserted, and reference numeral 900 denotes a propagation direction of laser light emitted by the laser diode chip 100.
FIG. 3 illustrates an example of a plan view of a TO can-type package having a diameter of 6.0 mm which is recently used.
As illustrated in FIG. 3, in the case in which the laser diode chip 100 is assembled using the TO can-type package, in order to perform emission of laser light at a central portion of the TO can-type package, the 45° reflection mirror 600, the lens 500, and the laser diode chip 100 should be concentrated at one side of the TO can-type package. In the case of TO 60 standard having a diameter of 6.0 mm, an internal diameter allowed in any one direction from the center of the package is only about 1.9 mm. A half of the 45° reflection mirror 600, the lens 500, and the laser diode chip 100 should be disposed in the above-mentioned length, and in the case in which a length of the 45° reflection mirror 600 of at least about 0.5 mm is included and the thickness of the lens 500 is typically allocated to about 0.8 mm, a width allowed for the sub-mount 200 of the laser diode chip 100 is only 0.6 mm.
Meanwhile, in order to operate the laser diode chip 100 at high speed, a standard of all signal lines transmitting a signal to the laser diode chip 100 may be impedance-matched to single ended impedance of each of positive/negative transmission lines of 25 ohms, respectively, and may be impedance-matched to differential ended impedance of 50 ohms in the case in which two positive/negative transmission lines are incorporated. In FIG. 3, reference numeral 202 denotes a line for a differential ended signal transmission.
FIG. 4 illustrates an example of a single transmission line, and FIG. 5 illustrates an example of a differential impedance transmission line. In a structure of FIG. 4, when a silicon substrate having a dielectric constant of 11.4 and a thickness of 0.4 mm is used, single ended impedance values calculated according to a width W of the transmission line 201 are shown in FIG. 6. In FIG. 6, in order to perform an impedance-matching of 25 ohms, the width W of the transmission line 201 should be about 1.05 mm. Since this width exceeds 0.6 mm, which is a size allowed as a width of a transmission substrate 200 for a laser diode chip in a very narrow TO can-type laser diode package; consequently it becomes the transmission line having a standard that cannot be adopted for a TO can-type subminiature package.
In addition, in a structure of FIG. 5, when the silicon substrate having the dielectric constant of 11.4 and the thickness of 0.4 mm is used, impedance values of the differential ended transmission line 202 calculated according to the width W of the transmission line 202 and a distance S between the transmission lines are shown in FIG. 7. In FIG. 7, in order to perform an impedance-matching of 50 ohms, a width of one transmission line 202 should be at least about 0.45 mm, and consequently, in the case of the differential ended transmission line requiring two transmission lines, a width of a substrate of a ultrahigh-speed transmission line should be at least 0.9 mm or more. Since this width exceeds 0.6 mm, which is a size allowed for the transmission substrate 200 for a laser diode chip in the very narrow TO can-type laser diode package, it becomes the transmission line having a standard that may not be adopted for the TO can-type subminiature package.